[ARC5] Resistor Excursions

[Tom Lee]

My measurements agree with what Jacques and Richard have said. I once 
characterized a junkbox full of CC resistors on a network analyzer in 
order to derive the simplest model that would be reasonably valid out to 
~1 GHz and easy for my students to remember when choosing components. 
Out of that effort I came up with a crude rule of thumb: A CC resistor 
acts very much like a resistor shunted by a capacitance whose value in 
pF is numerically equal to the power rating in watts. So, a half-watt 
resistor has about a half-pF capacitance, and so on. It's not a perfect 
fit, but in terms of "bang for the buck" it serves well. It satisfied 
the main goal of the exercise, to come up with something easy to remember.

As I recall, film resistors were quite similar; the inductive nature 
that many expect because of the spiral construction didn't really show 
up unless the resistances were fairly low (~100 ohms on down). The 
capacitance tended to dominate the reactance.

The same weekend of experiments yielded another crude rule of thumb: The 
shunt capacitance of a surface-mount resistor is (again very 
approximately) in pF the size designation of the part, with a leading 
decimal point. So, an 0805 resistor has about 0.08pF capacitance 
shunting it, etc.

These approximations aren't intended for use in circumstances where high 
accuracy is needed. They're just for answering questions like, "Would 
this resistor likely work in this part of this 1 GHz amplifier?"

-- Cheers,
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 4/19/2024 7:45 PM, Jacques Fortin wrote:
> I agree with Richard comments.
> I also measured a lot of CC resistors with Impedance Analyser equipment and I do not agree with what KB8TQ states.
> The CC resistors turns capacitive at high frequency and the phase angle do not stay close to 0 degree (they did not stay "resistive").
> Depending on the part value, it progresses thru -90 degrees when the frequency increases.
> Those who are sceptical about this should take a look at the Allen Bradley resistors catalog for the year 1994.
> The behavior of CF, MF and MOF resistors is better, as they stay purely resistive way higher in frequency than the CC.
> Test wise, a MF resistor does not exhibit more inductance at RF frequencies than the equivalent length of straight wire.
>
> 73, Jacques, VE2JFE in Montreal
>
> What is the source of your information?  I have measured many CC resistors using a Boonton RX-Meter and found they become more reactive at RF. While many think film resistors become inductive I have not found that true. Curiously enough the examples in the Boonton handbook show the same thing.
> I was taught the same thing about CC resistors but it seems not to be true.
>
> Richard Knoppow
> Los Angeles
> WB6KBL
> SKCC 19998
>
> On 4/19/2024 4:23 PM, Bob kb8tq wrote:
>> Hi
>>
>> If you *do* decide to replace CC’s with a different type of resistors, do a deep dive into just how weird CC’s are at RF. Unlike pretty much all other resistors, they drop value, but stay resistive as frequency goes up. While that sounds a lot like a resistor model that includes a cap, it’s not. The phase shift with “pure resistive” is not the same ….
>>
>> How much does this matter? It very much depends on how tricky the circuit happens to be. In many cases … not a big deal.
>>
>> Bob
>>
> --
>
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